In conventional fastening systems, e.g. a screw driver and screw, the driver often slips out of the recess in the head of the screw, particularly when the recess is formed in the shape of a single groove or a "Phillips" head configuration. Slippage can occur during the last few turns in driving the screw into a workpiece, in starting the screw or taking it out, and, especially, if the recess of the screw, or the tip of the screwdriver, is worn. As a result of this slippage, axial pressure or end load must be applied to the screwdriver to prevent its driving end from repeatedly slipping out over the walls of the recess or groove in the screw head. But the application of substantial axial pressure or end load on the screw is very often undesirable, particularly in medical applications where bone screws are employed to repair fractures and other damage to bone. The application of a high end load to a bone screw can further damage already traumatized bone tissue and must be minimized. Additionally, if the screwdriver slips from the screw while applying a high end load force, the surface of the material containing the screw may be scratched and marred by the slipping of the screwdriver blade. This problem becomes quite serious in working with finished hardwood constructions, where the screwdriver may scratch or disfigure the work. Moreover, once a screwdriver has slipped from the recess of a screw, it becomes more difficult to prevent its recurrence. The more times the blade slips, the more rounded the recess walls become, making it even more difficult to screw or unscrew the screw.
Another aspect of fastening systems involves the problem of starting the insertion of the screw into a given material. With a conventional screwdriver, an operator usually holds the screw in one hand while operating the driving tool with the other. It may not always be possible or efficient to pre-drill the screw hole, and/or the location where the screw must be inserted may make this starting operation difficult. As a result, the screw is often repeatedly dropped, requiring reapplication and wasted effort.
Interlocking driver and fastener systems have been developed to solve problems of the type mentioned above. U.S. Pat. No. 4,339,971, for example, discloses a driving tool and fastener in which the head of the fastener is formed with a generally inverted T-shaped recess or slot and a rectangular-shaped bit is formed at the driving end of the driving tool. When the bit of the driving tool is placed in engagement with the head of the screw, opposite corner edges of the bit enter and wedge into contact with one or more undercuts formed in the screw head by the recess. These undercuts or cutouts each include a tapered upper wall engageable with the corners of the driver bit along a limited surface area, e.g. no more than line contact is provided therebetween. As a result, when the driver bit wedges into position on the screw head, substantial wear is created in both the driver and screw because they only contact and transfer torque between one another along a relatively small area.
Other systems have complementary obliquely angled wedge-type surfaces on the driver and fastener. U.S. Pat. Nos. 132,946; 1,056,095; 2,304,704; 2,684,094; and 4,311,071; and Canadian Patent No. 1,167,293, for example, generally disclose a non-slip screwdriver and screw head system wherein the sides of the screw head recess and the sides of the driver are angled complementarily and obliquely in relation to the axis of the driver and the screw head. These two wedge-like surfaces form an oblique, interlocking couple when the driver and screw engage one another. While the driver and fastener of interlocking systems of the type disclosed in the above-listed patents form a large area of contact between the sides of the driver and the walls of the fastener head, they do not form a stable interlocking system. As a result, the driver and screw can disengage one another, and, in certain alignments, the fastener tends to slip from the driver.
Another fastener system is disclosed in U.S. Pat. No. 4,936,172 to Jackson, which comprises a fastener head formed with a pair of undercuts on opposite sides of a central slot within which the rectangular shaped driving end of a driver is insertable. Each of the undercuts includes a sidewall and shoulders which extend outwardly from the respective sidewalls. The driving end of the fastener includes an outermost extent formed with opposed notches or grooves defining flange regions at the base of the outermost extent. The blade of the driver slidably mounts a sleeve which carries longitudinal extensions on opposite sides thereof. In order to utilize the fastener system disclosed in the Jackson Patent No. 4,936,172, the driving end of the driver is inserted within the central slot of the fastener head and rotated in a single direction such that the shoulders of the undercuts are captured within the grooves in the driving end of the driver. Operative engagement of the driver and fastener is achieved by thereafter sliding the sleeve downwardly along the driver blade until the longitudinal extensions engage the base of the slot and the fastener head. The longitudinal extensions fill the void areas of the slot and the fastener head, and they exert a downwardly directed force which wedges the flanged regions of the driver upwardly against the shoulders of the undercuts in the fastener head.
The construction of the Jackson '172 fastener system is extremely complicated and difficult to manufacture. Locking between the driver and head of the fastener is dependent upon the use of a sliding sleeve and longitudinal extensions in order to ensure that the driver and fastener do not disengage as the fastener is being inserted into a workpiece. This construction is expensive and is susceptible to failure after an extended period of use.